A study led by paleoanthropologists Daniel García-Martínez (CENIEH, Burgos) and Markus Bastir (Museo Nacional de Ciencias Naturales, CSIC; Madrid) published in the journal Nature Ecology and Evolution, reveals for the first time what the ribcage of the famous Homo erectus skeleton, known as the Turkana Boy, looked like.

Surprisingly, it had a deeper, wider and shorter chest than seen in modern humans, suggesting that Homo erectus had a stockier build than commonly assumed based on its reputation as the earliest long distance runner among our ancestors with a lean body to match. It thus appears that the fully modern human body shape evolved more recently than previously thought, rather than as early as two million years ago when Homo erectus first emerged.

The evolution of modern human body shape is interesting in light of the way we and our ancestors are adapted to our natural environment. As modern humans, Homo sapiens, we have a relatively tall, slender body shape that contrasts with, for example, the shorter, stocky, heavy bodied Neandertals.

Scientists have long assumed that our body shape originated with the first representatives of Homo erectus in the context of climate changes and the receding forests in tropical Africa, over two million years ago. Our tall and slender bodies seem evolutionarily advantageous in the expanding hot and dry savannah, helping to avoid overheating and well suited to bipedal running over long distances in more open terrain.

Fossils attributed to Homo erectus accordingly point to longer legs and shorter arms than our earlier ancestors, the australopiths, which were bipeds when on the ground, but still retained some commitments to life in the trees. Several modern body characteristics are particularly clear in the 1.5 million year old fossil remains of a Homo erectus adolescent found just west of Lake Turkana, Kenya. Known as the Turkana Boy, it is the most complete skeleton of a fossil human ancestor ever found.

Studies of how this individual walked and ran have largely been restricted to the skeleton’s legs and pelvis. However, for endurance running its breathing capabilities would have been relevant as well. Thus far this aspect has not been investigated in any detail because assessing the chest and breathing motion based on a jumble of rib and vertebra fossils is difficult with conventional methods. Now, with the introduction of increasingly sophisticated imaging and reconstruction techniques this study has at long last become possible.

In the new research a three-dimensional virtual ribcage of the Turkana Boy could be reconstructed, and its adult shape could be predicted had this adolescent fully grown up. The ribcage shape was compared with that of modern humans and a Neanderthal, and virtual animation allowed breathing motion to be investigated.

“The results are now changing our understanding of Homo erectus”, says lead author Markus Bastir, "its thorax was much wider and more voluminous than that of most people living today". Adds second author Daniel García Martínez: “Actually, the ribcage of Homo erectus seems more similar to that of more stocky human relatives such as Neanderthals, who would have inherited that shape from Homo erectus”.

“Our own body shape with its flat, tall chest and narrow pelvis and rib cage, likely appeared only recently in human evolution, with our species, Homo sapiens” says Scott Williams (co-author of the study at NYU, New York). The paper speculates that these changes to our body shape may have optimized breathing capabilities for long-distance running and other endurance activities. Concludes Fred Spoor (Natural History Museum, London, senior author of the study): “That Homo erectus was perhaps not the lean, athletic long-distance runner we imagined is consistent with more recent fossil finds and larger body weight estimates than previously obtained; this iconic ancestor was probably a little less like us than we portrayed it over the years”.